feat(api): retire phase-1/2 /workstreams REST aliases with 410 stranglers

Add retire_legacy_route() helper and convert CRUD, dependency, and
execution intent legacy routes to 410 Gone while preserving final-call
legacy-meter attribution. Update tests, retirement backlog, and close
STATE-WP-0075 with handoff to STATE-WP-0070 T02 and STATE-WP-0073 T04.

workplans/STATE-WP-0075-workstream-route-410-stranglers.md

@@ -0,0 +1,94 @@
+---
+id: STATE-WP-0075
+type: workplan
+title: "Workstream REST route 410 stranglers"
+domain: infotech
+repo: state-hub
+status: finished
+owner: codex
+topic_slug: custodian
+planning_priority: high
+planning_order: 75
+created: "2026-07-10"
+updated: "2026-07-10"
+---
+
+# STATE-WP-0075 — Workstream REST route 410 stranglers
+
+**Parent:** `STATE-WP-0070` T02 (phase-2 legacy retirement).  
+**Handoff from:** `STATE-WP-0073` T04 (meter-cleared REST key retirement).
+
+**Baseline evidence:** `docs/evidence/legacy-meter-weekly-review-20260710.json`
+— phase-1 `/workstreams/*` keys at 0 calls (retirement candidates; ~2/7
+consecutive 8h zero windows at capture time). Operator approved early 410 rollout
+after caller remediation (`STATE-WP-0072`) drove hot-key counts to zero.
+
+## Goal
+
+Convert remaining phase-1 and phase-2 `/workstreams` REST aliases to **410 Gone**
+stranglers that still meter final legacy usage, matching the existing
+`DELETE /workstreams/{id}` and `GET /workstreams/workplan-index` pattern.
+
+Preferred `/workplans/*` routes remain the only functional surface.
+
+## Task: Shared retire_legacy_route helper
+
+```task
+id: STATE-WP-0075-T01
+status: done
+priority: high
+```
+
+1. Add `retire_legacy_route()` to `api/services/legacy_compat.py` — meter usage,
+   set deprecation headers, raise `HTTPException(410)`.
+2. Refactor existing 410 routes (`workplan-index`, `DELETE`) to use the helper.
+
+Done when all 410 stranglers share one code path for metering and headers.
+
+## Task: Phase-1 workstream CRUD stranglers
+
+```task
+id: STATE-WP-0075-T02
+status: done
+priority: high
+```
+
+Return **410 Gone** on:
+
+- `GET /workstreams/`
+- `POST /workstreams/`
+- `GET /workstreams/{workstream_id}`
+- `PATCH /workstreams/{workstream_id}`
+
+Keep `/workplans/*` handlers unchanged.
+
+## Task: Phase-2 dependency and execution stranglers
+
+```task
+id: STATE-WP-0075-T03
+status: done
+priority: high
+```
+
+Return **410 Gone** on:
+
+- `POST/GET/DELETE /workstreams/{workstream_id}/dependencies/…`
+- `PATCH /execution/workstreams/{workstream_id}/intent`
+
+## Task: Tests, backlog, and parent handoff
+
+```task
+id: STATE-WP-0075-T04
+status: done
+priority: high
+```
+
+1. Update `tests/test_legacy_meter.py` and `tests/test_routers_core.py` for 410
+   expectations on all retired keys.
+2. Mark phase 1–2 REST keys retired in
+   `docs/workplan-terminology-legacy-retirement-backlog.md`.
+3. Run `pytest tests/test_legacy_meter.py tests/test_routers_core.py`.
+4. Register via `statehub fix-consistency --repo state-hub`.
+5. Hand off completion to `STATE-WP-0070` T02 and close `STATE-WP-0073` T04.
+
+Done when tests pass, workplan registered, and parent tasks reflect retirement.